Since it still hasn't shown up in the Settings app in the current Redstone 4 release candidate, build 17134, you probably assumed that S mode will work exactly as it does now, and you'd be right. As it turns out, you'll be able to opt out of S mode and upgrade to a full version of Windows through the Microsoft Store, and you'll need either a full Windows image or an installer to get back into S mode.

As spotted by Rich Hay, a 'Switch out of S Mode' listing has shown up in the Microsoft Store. It even seems to be searchable on the current production version of Windows 10, including on Home and Pro SKUs where there's nothing to switch out of. Unless you actually have a PC that's in S mode though, clicking on the listing will fail to load the page. If you are on S mode, you'll see the following:

Here is the "Switch out of S Mode" page in the Microsoft Store on Build 17134. Shows the free switching option that was previously announced. pic.twitter.com/SPT8MS52wB

The listing seems to just be a reworked version of the Windows 10 Pro page that you'd see when upgrading from Windows 10 S. The feature list hasn't changed. You should see a different list of features if you're switching between Windows 10 Home and Home in S mode though.